I have a similar strip. The red leds have a fwd voltage of 2, and use the 330 ohm resistors per 3 leds. Blue and green are 3V and each use a 150 ohms resistor. Use http://led.linear1.org/led.wiz to tinker with this. Now, about the amperage I made same calc as you did to arrive at 60mA*50 = 3A. Either the supply is beyond max or it isn't lighting the LEDs properly. In any case, it's a shame they didn't make the strings twice as long, in which case they could have left out the resistor on the red leds. Now, how do drive these with my arduino ?